The present invention relates to a static pressure gas bearing apparatus for turbocharger which is to be placed on an internal combustion engine.
In general, a turbocharger for an internal combustion engine which supercharges the suction gas by utilizing the pressure energy of the exhaust gas is rotated at high speed so that reducing the shaft friction is a major factor which is responsible for improving its performance.
With this in mind, the present applicant proposed a bearing apparatus which supplies pressurized air as the shaft lubricating medium of the turbocharger (see Japanese Laid-Open Utility Model No. 60-55738).
The apparatus has a static pressure gas bearing 1 with a configuration that supports a rotating shaft 2 of the turbocharger in floated state by supplying pressurized air to the space between the rotating shaft 2 and a bearing unit 4, as shown in FIG. 1, and air that is stored under pressure in an accumulator is supplied to the static pressure gas bearing 1 through a pressurized air supply passage 5.
Disposed midway in an air conduit 22 that connects the accumulator 7 to an air pump 8 which is driven either mechanically by the engine or by an electric motor, is a check valve 9, and analogously disposed midway in an air conduit 21 that connects the accumulator to the static pressure gas bearing 1 is a regulating valve 6.
The regulating valve 6 is opened by a signal from the ignition switch and supplies air that has been accumulated in the accumulator 7 prior to the start of the engine to the bearing unit 4. On the other hand, when the engine is brought to a stop, it supplies air stored in the accumulator 7 to the bearing unit 4 for some time after the stop of the engine, by means of the timer function incorporated in the ignition switch, to avoid contact of the rotating shaft 2 of the turbocharger with the bearing unit 4.
Now, the pressurized air has to be supplied to the bearing unit 4 on two occasions, namely, immediately after the interruption and at the time of restarting of the engine, in the state in which the engine operation is interrupted, so that the volume of the accumulator 7 has to be made large in size. However, when the accumulator 7 is to be placed on a motor vehicle or the like, it leads to the difficulty of securing a sufficient internal pressure for the accumulator 7 due to the restriction on the available space that is limited.
Further, if by any chance a minute leakage of accumulated air occurs in the accumulator 7 or somewhere in the air conduits 21 and 22, and if it is overlapped with an operating condition such as a rapid acceleration immediately after the restarting of the engine, then there arises a fear of generating burning due to contact of the rotating shaft 2 of the turbocharger with the bearing unit 4 without receiving sufficient amount of air on the surface of the bearing.